Non-aqueous electrolyte secondary batteries (typically lithium ion batteries) have recently become increasingly important as high output power supplies for vehicles and power supplies for electric power storage systems because of the light weight and high energy density thereof. In order to further increase the input and output power, reduction in internal resistance of the batteries has been attempted.
A typical configuration of the type of the non-aqueous electrolyte secondary battery includes electrodes (a positive electrode and a negative electrode) carrying electrode active material layers containing electrode active materials on electrode current collectors. The electrode is typically formed by applying a paste for forming an electrode active material layer containing the electrode active material, a binder and the like on the surface of the current collector, drying the layer and then pressing the layer so as to obtain a predetermined density (a so-called application method).
Widely known negative electrode active materials included in negative electrodes may include graphite materials including natural graphite, artificial graphite and amorphous carbon of natural graphite or artificial graphite. Such graphite materials have laminar structures containing a plurality of planes formed with carbon six-membered rings (which are also referred to as graphene and correspond to the (002) planes in the graphite crystal structure) stacked together. Charge and discharge are the results of insertion (storage) of lithium ions between the layers (interlaminar) and elimination (release) from the interlaminar space. Various devices have been proposed in order to improve performances of negative electrodes containing such graphite materials as a negative electrode active material.
For example, Patent Literature 1 discloses orientation of a negative electrode active material of a graphite type by applying a magnetic field to a negative electrode active material composition layer containing the negative electrode active material while the layer is in a viscous state. It is disclosed that, according to this technique, lithium can migrate through the interlaminar spaces of the oriented graphite via a substantially linear route or an approximated route thereof and the graphite existing inside of the negative electrode active material layer can contribute to charge and discharge, resulting in an improvement in battery charge and discharge capacity. It has been known that, when graphite is arranged so that the plane direction of the graphite is perpendicular to a current collector for example, lithium can be smoothly inserted to and eliminated from the interlaminar spaces of graphite, resulting in reduction in resistance and an improvement in output and input properties.
Patent Literature 2 discloses a negative electrode active material which is a composite carbon material containing a graphite material obtained by heat treatment of coke and vapour grown carbon fibre (VGCF). It is described that according to this configuration, swelling of electrodes can be further prevented compared to the electrodes containing only the graphite material upon charge and discharge and an increased capacity can be realized compared to the electrodes containing only the VGCF, resulting in provision of negative electrodes for secondary batteries having excellent cycle properties, high voltage and high capacity.